1. Field of the Invention
The present invention relates to a temperature compensation circuit used in an apparatus for negative-impedance driving a load and, more particularly, to a temperature compensation circuit used for efficiently compensating for a variation in drive state caused by a change in temperature of a load.
2. Description of the Prior Art
In general, an electro-magnetic transducer (dynamic electro-acoustic transducer) such as a speaker has a coil (voice coil) disposed in a magnetic gap of a magnetic circuit, and a drive current i flows through the coil to drive a diaphragm or the like. The coil has an inherent internal impedance including a DC resistance component.
Since the coil formed of a copper wire has a positive temperature coefficient, its resistance changes depending on a temperature. If the length of the copper wire coil is presented by l, and the intensity of the magnetic field of the magnetic gap is represented by B, a drive force F appearing at the copper wire coil is given by: EQU F=B.multidot.l.multidot.i
Therefore, in the case of constant-voltage driving, the drive force changes depending on a temperature. The above-mentioned electro-magnetic conversion system generally has a motional impedance, and the resistance component of the copper wire coil serves as a damping resistance of this motional impedance. Therefore, a damping force also changes in accordance with a change in temperature.
In a speaker system provided with the speaker as the electro-magnetic conversion system described above, the internal impedance inherent in the voice coil of the speaker seriously influences the resonance Q value and the lowest resonance frequency f.sub.O of sound pressure characteristics.
The present applicant paid attention to the above fact, and filed a patent application for an acoustic reproduction apparatus which can equivalently invalidate or eliminate a DC resistance component of the voice coil (U.S. patent application Nos. 286,869 and 287,381, and the like; not disclosed). FIG. 6(a) is an equivalent circuit diagram showing the concept of the above-mentioned apparatus. In FIG. 6(a), reference symbols C.sub.M and L.sub.M denote a capacitance component and an inductance component of a motional impedance Z.sub.M of an electro-magnetic transducer (speaker), respectively; and R.sub.V, an internal resistance of a voice coil as a load. The internal resistance R.sub.V is eliminated by a negative resistance -R.sub.A equivalently formed at the driving side, and an apparent drive impedance Z.sub.A is given by: EQU Z.sub.A =R.sub.V -R.sub.A
If Z.sub.A becomes negative, the circuit operation is rendered unstable. Therefore, the values of R.sub.V and R.sub.A are set as R.sub.V .gtoreq.R.sub.A.
According to this acoustic reproduction apparatus, the resonance Q value of a unit vibration system constituted by a speaker ideally becomes zero, and the concept of the lowest resonance frequency f.sub.O is also lost. When the above resonator is driven, since the resonance Q value need not be decreased, a strong resonance acoustic wave radiation can be realized.
In order to achieve such negative-impedance driving, the negative impedance must be equivalently generated. For this purpose, a detection element R.sub.S as a current detection element 3 is connected in series with the speaker as a load 2. FIG. 6(b) is a circuit diagram of a negative impedance generator. As shown in FIG. 6(b), the detection resistor R.sub.S is connected to the load 2 (internal resistance R.sub.V), and its detection output is supplied to an adder 5 through a feedback circuit 4 of a feedback gain .beta. to be positively fed back to an amplifier 1 of a gain A. Therefore, an equivalent output impedance R.sub.O with respect to the load 2 is given by: EQU R.sub.O =R.sub.S (1-A.multidot..beta.)
However, in the conventional method, the following problems are posed.
In the negative-impedance driving system, both the large drive force and damping force can be realized. However, unless appropriate temperature compensation is performed for a change in DC resistance R.sub.V of the voice coil caused by a change in temperature of the voice coil of the speaker, a drive state varies more largely than in the case of the normal constant-voltage driving. In the negative-impedance driving system, it is difficult to make constant the drive impedance with respect to the motional impedance of the voice coil or the like of the speaker over a wide temperature range. For example, in the circuit shown in FIG. 6(a), since the drive resistance is (R.sub.V -R.sub.A), if the equivalent negative resistance -R.sub.A is set to be constant regardless of a temperature, the ratio of influence of a change in resistance R.sub.V caused by a change in temperature with respect to the drive state becomes larger than that in the case of the constant-voltage driving. There is no conventional means for positively compensating for a change in temperature of the internal resistance R.sub.V.
In some apparatuses, a coil for detecting the temperature of a drive coil as a load formed of the same material (e.g., copper) as the drive coil is attached near the drive coil. The coil for detecting the temperature does not serve as a normal coil, and is merely utilized as a resistor whose resistance changes depending on a temperature.
In a method wherein the coil (detection coil) for detecting the temperature is used in addition to the coil as the load, however, an extra terminal for the detection coil is required. Since the mass of the entire coil is increased by the detection coil, if it is applied to the speaker, the vibration system itself undesirably becomes heavy. Furthermore, wirings to the speaker unit cannot be realized by two terminals, thus disturbing compatibility.